Effect of Travel on Sleep Patterns and Athletic Performance in Female Professional Tennis Players: A Retrospective Cohort Study Utilizing WHOOP 3.0 Tracking

Abstract

Background:

Sleep is vital for an athlete’s recovery, physical and mental health, and athletic performance. The impact on circadian rhythm from long-distance travel across multiple time zones has not been studied using wearable technology in female professional tennis players.

Hypothesis:

Travel between time zones in female professional tennis players leads to circadian desynchronization, causing disruption in sleep patterns, changes in physiologic parameters, and decreased athletic performance.

Study Design:

Retrospective cohort study.

Level of Evidence:

Level 3.

Methods:

A total of 52 female professional tennis players consented to wear, and share the data of, WHOOP 3.0 while traveling and competing on the Women’s Tennis Association (WTA) Tour. Linear mixed models examined the relationship between (1) travel and sleep/recovery and (2) sleep/recovery and match performance.

Results:

Sleep duration without travel averaged 437 minutes (436.8 ± 2.8). On the first night after travel, for every hour time zone difference (TZD) traveled regardless of direction, players slept 11 minutes less (−11.3 ± 0.96; P < 0.05). Eastward travel further reduced sleep (−24.5 ± 4.2; P < 0.05), while westward travel increased sleep duration (+30.0 ± 4.2; P < 0.05). These effects were reduced on the subsequent 2 nights.

Conclusion:

Sleep duration reduction was most prominent on the first night after travel, particularly with eastward travel, but improved on subsequent nights. The number of time zones crossed predictably increases the vulnerability for insufficient sleep duration, whereby a 2-hour TZD easterly, and 4-hour TZD westerly, reduces sleep duration <7 hours on the first night. No significant correlation was found between sleep disruption and competition performance in our female professional tennis population.

Clinical Relevance:

The findings suggest that professional tennis players should, and generally do, arrive at competition locations with enough time to resynchronize their circadian rhythm to the destination time zone; particularly if traveling eastward.

Quality sleep plays a significant role in an athlete’s overall physical and mental health, recovery, injury risk, and athletic performance. ³² The circadian system regulates the daily cycle between sleep and wakefulness, but also plays a role in affecting physiological and behavioral processes over a 24-hour period. ¹⁸ Human exercise performance, whether speed, strength, or endurance, has been shown to have a nadir in the early morning and peak in the late afternoon/early evening as a result of neurological, hormonal, and muscle-specific factors regulated by the circadian rhythm.^12,17 An athlete’s optimal daily training schedule for peak performance can be interrupted by travel, which has become an increasingly important part of elite sports as competitions across the world require frequent travel throughout the year. The extent and direction of the travel exposes the elite player to circadian desynchrony, the misalignment between their internal circadian rhythm and the local time at their new destination. This results in physiologic symptoms including daytime fatigue, sleep disruption, reduced concentration and alertness, and gastrointestinal disturbances, which may in turn contribute to increased risk of illness, injury, and decreased athletic performance.^18,44 The term “jet lag” is used to describe a constellation of symptoms after rapid travel across ≥3 time zones, with symptoms more severe and prolonged compared with travel fatigue in which symptoms are caused not by significant circadian desynchronization but rather the demands of travel itself. ¹⁸

Founded by Billie Jean King in 1973, the Women’s Tennis Association (WTA) is a unique entity in professional sports where the world’s best female tennis players travel across the globe year-round to compete at the highest echelon of competitive tennis. The annual WTA Tour engages players in >50 tournaments and 4 Grand Slam events traversing 6 continents and 30 countries with varying time zones. To optimize performance, it is imperative that players plan their travel and tournament schedule with jet lag recovery in mind. In 1995, the WTA introduced the Age Eligibility Rule (AER) in conjunction with a mandatory Player Development Programme (PDP), a novel initiative providing education and guidance for young rising professional tennis players, including monitoring of their schedules, with the ultimate goal of prolonging their tennis careers safely. The 10- and 25-year outcome data was incredibly positive with significantly improved career longevity postimplementation of AER/PDP with a career duration of 14.2 years post-AER/PDP versus 12.1 years pre-AER/PDP. ²⁹

Research examining the impact of sleep on athletic performance, mental health, injury risk, and recovery continues to grow. The prevalence of lower quantity and quality of sleep is higher in athletes than nonathletes. ⁶ Especially surrounding competitions, sleep quality may deteriorate further due to training load, potential travel and sleep in unfamiliar settings, and/or stress and anxiety about the event. ²¹ The International Olympic Committee’s consensus statement on mental health in elite athletes recognizes the importance of sleep on an athlete’s overall health and performance. ³¹ Insufficient sleep, identified as <7 hours per night, leads to poor recovery and increased risk of overtraining syndrome, illness, and injury, in addition to fatigue and other potential mental health disorders. ³¹ The National Collegiate Athletic Association also published consensus recommendations regarding the impact of sleep on the collegiate athlete - a very similar age to our WTA cohort. ²²

Thun et al ⁴² provided a systematic review of various studies that examined the impact of sleep on performance in athletes, including circadian desynchronization from travel. In general, it is accepted that eastward travel can be more disruptive to sleep patterns. Data over 40 years in American professional football have demonstrated that United States west coast football teams performed better when compared with east coast teams when the game time was restricted to an evening game played away on the east coast, for example Monday Night Football. ³⁵ The rationale for this difference was due to the west coast team having the game time better aligned with their internal circadian rhythm’s afternoon performance peak. For example, an east coast game start time of 7:00 pm eastern standard time is equivalent to 4:00 pm Pacific standard time. ⁴² Retrospective analysis of 2014 to 2018 National Basketball Association data found that circadian desynchrony and distance traveled negatively influenced performance, and these effects can be heightened when disadvantageous scheduling such as back-to-back games are involved. ⁷ Because of the small margin of difference between success and failure in professional sports, particularly individual sports such as tennis, it is crucial for elite athletes to perform at their physical and mental peaks. This includes implementing strategies to realign circadian rhythm for recovery to help maximize physical and cognitive performance.

Despite the advancement in research related to travel, sleep, and circadian rhythm disruption on elite athlete performance, the existing research has been limited by lack of practical in-the-field technology to track sleep. Limitations to previous studies with similar aims include lack of ability to measure sleep changes to help establish circadian rhythm. Typical measurements to establish circadian rhythm include either measuring melatonin directly (via blood, saliva or urine) or tracking core body temperature, ¹⁸ both of which become impractical in the traveling athlete population outside of a laboratory. To advance this body of literature, our project used WHOOP 3.0, a validated wrist-worn biometric tracking device to provide insight on the effect of travel on sleep quality, physiologic changes, and athletic performance at the individual level. ²⁷

Heart rate variability (HRV)—a measurement made possible by wearable technology such as WHOOP 3.0, is a well-established measure of autonomic nervous system function, and has been a focus of recent research with broad applications, ranging from a measure of recovery after physical training,^10,30,37 to response to acute social stressors,^16,30 to assessment of tactical personnels’ readiness to perform. ³⁸ In general, high HRV reflects a well-adapted autonomic function and is associated with better recovery and athletic performance, whereas a decrease in HRV from baseline can indicate stress or fatigue.^26,30 A small number of studies have also demonstrated that travel-related stress can affect HRV negatively.^{3,11,13,14,40,41} Sleep deprivation, whether travel-related or not, is another factor known to reduce HRV. ¹

There are currently limited studies that have used wearable devices to investigate the effect of travel on sleep and physiologic parameters and its potential influence on athletic performance. In addition, as in many other areas of sports medicine research, there is a paucity of literature dedicated to the investigation of female athletes. The aim of this project is to evaluate the impact of travel-related circadian desynchronization and its correlation with altering sleep and physiologic recovery patterns, with a secondary aim to evaluate how these impacted variables may affect athletic performance in elite female tennis players. While it is impossible to control for all the individual and environmental factors that may affect the influence from travel, by pairing the number of time zones crossed to our dataset, we hope to observe general trends that illustrate the impact of circadian misalignment due to travel. We hypothesize that travel across time zones in female professional tennis players leads to circadian desynchronization causing disruption in sleep patterns, stress responses in physiological parameters, and decreased athletic performance.

Methods

Our retrospective study population included 52 female professional singles tennis players with full or associate membership on the WTA tour who are ≥18 years of age and have consented to use the WHOOP 3.0 wrist-worn biometric capture device. Players are eligible for full membership status if they finish on the WTA year end rankings in the top 150 in singles or 50 in doubles and play a minimum of 6 WTA 250 level and above tournaments. Associate members must have played in 3 WTA 250 level and above events in the last 12 months and be ranked inside the top 500 singles or 175 in doubles. The WHOOP 3.0 device is intended to be worn continuously by the player to monitor sleep, recovery, training, and performance. To be included in our study, the player must have worn WHOOP 3.0 for a trip with at least 5 time zone changes. Although the player has the ability to utilize their personal data, they also consented for data to be accessed by WHOOP and the WTA for use in a deidentified manner, which includes research purposes. As deidentified data was used, our study was found exempt from our organization’s institutional review board as it does not constitute research involving human subjects as defined under 45 CFR 46.102. We then evaluated sleep-related data provided by WHOOP 3.0 for our target population retrospectively before and after traveling to professional WTA tournaments, with performance data analyzed from tournaments that occurred from March 1, 2022, through November 30, 2023.

Outcomes included analysis of athletic performance for those consented players, obtained from Stats Perform—a commercial statistics provider for the WTA—and was aggregated for each singles match played. We compiled first serve percentage, double fault frequency, ace frequency, total points percentage, total winner frequency, and total unforced errors frequency. Although not actual measures of performance, we also recorded the player’s current rank and current rank difference to the opponent to control for discrepancy in player skill during matches. Other potential confounding factors, such as environmental temperature, light exposure, altitude, wind, and humidity at the destination of travel and tennis specific factors such as court surface, were not controlled due to the variability of these factors.

Selected sleep quality and physiologic measurements used in our study obtained from WHOOP 3.0 included sleep duration, sleep consistency, resting heart rate (RHR), HRV, HRV-coefficient of variation (HRV-CV), respiratory rate (RR), skin temperature, and oxygenation (SpO₂) (Table 1). Data were extracted from WHOOP 3.0 devices as a daily summary, with no preprocessing or filtering applied before analysis.

Table 1.

Descriptive statistics of each sleep and recovery variable captured by the WHOOP 3.0 device

Variable	Definition	Mean ± SD
Sleep duration, minutes	Time from sleep onset to waking	439.7 ± 81.7 (7.33 hours)
Sleep consistency, %	Uniformity of sleep and wake times over previous 4 nights, depicted on a 0-100 scale with 100% representing perfect uniformity of sleep. Value is determined by WHOOP proprietary calculations	71.2 ± 15.4
Resting heart rate, bpm	Heart rate while at rest during sleep	52.0 ± 6.4
HRV, ms	Changes in time between successive heartbeats as measured by R-R interval differences	88.4 ± 34.7
HRV-CV	Statistical biomarker that measures long-term adaptation to training load; lower HRV-CV means greater capacity to take on training load	19.2 ± 11.4
Respiratory rate, rpm	Number of respirations per minute	16.0 ± 1.4
Skin temperature, °C	Cutaneous temperature	34.2 ± 0.7
SpO2, %	Blood oxygen saturation	96.1 ± 1.8

bpm, beats per minute; HRV, heart rate variability; HRV-CV, HRV-coefficient of variation.

WHOOP 3.0 is a wrist-worn biometric capture device that acts as a tool to help athletes maximize their potential by providing personalized data to track sleep changes, strain, and recovery. This device helps athletes have a better understanding of their physical limits, allowing them to learn patterns of when best to compete and when to recover. A previous partnership between the WTA and WHOOP permitted athletes to wear the device both on and off the court, serving as a research and learning tool to advance their health and wellness. ⁴⁶ WHOOP 3.0 uses continuous calibration of each athlete’s unique physiology to keep the most accurate data possible. WHOOP 3.0 tracks numerous data points including sleep duration, sleep consistency, RHR, HRV, HRV-CV, respiratory rate, skin temperature, and SpO₂. A recent validation study comparing 6 commercial wearable devices, including WHOOP 3.0, against gold standard measurement of sleep by polysomnography and electrocardiography, as well as heartrate and HRV during sleep, confirms the validity of WHOOP 3.0 in assessing sleep onset and duration. Furthermore, WHOOP 3.0 achieved the highest accuracy in measurements of heartrate (within 0.3 beats per minute) and HRV (within 4.5 milliseconds) out of the commercial devices. ²⁷ WHOOP 3.0 has been further validated to be used confidently by sport and exercise practitioners to record heartrate and HRV in practical settings over the course of a training block to assess performance readiness. ¹

Time zone variance and travel direction were determined according to sleep period data captured by the WHOOP 3.0 device. A time zone change was defined as occurring when 2 consecutive sleep periods were recorded in different time zones. Time zone difference (TZD) was calculated as the difference in local coordinated universal time offset between each time zone, in hours, and was represented as an absolute difference. For example, as London is considered time zone 0, thus New York is −5:00 and Melbourne is +10:00; with an absolute TZD of 15 hours between Melbourne and New York. This may be adjusted for day-light savings time depending on location and time of year.

Statistical Analysis

All statistical analysis was performed with the R programming language in RStudio (Version 2023.12.0). For all testing, significance was set at P < 0.05. Due to the hierarchical structure of the data, linear mixed models were used to analyze relationships between travel, sleep, and recovery metrics. To determine the influence of travel on sleep and recovery, separate linear mixed models were run in a univariate manner where one sleep/recovery variable was estimated according to TZD or travel direction, until all combinations of variables were explored in a multivariable linear mixed model. Random intercepts were used, with fixed effects defined as TZD and travel direction, and random effects defined as each participant. For any models which showed significant results, TZD and travel direction were then included in a single model to perform a multivariable analysis.

To determine the influence of sleep/recovery on match performance, data included in analysis were delimited to periods recorded between 1 and 3 nights before a match. Univariate linear mixed models were performed which separately analyzed the relationships between each sleep/recovery variable and each performance metric. Random intercepts were used, and each sleep/recovery variable was considered a fixed effect while participants were considered random effects. For models that produced significant results, these variables were analyzed together in a linear mixed model to determine multivariable effects.

Results

Influence of Travel on Sleep and Physiologic Parameters

The sample group included 52 female professional tennis players, mean age 27.8 ± 4.4 years, with 20,293 nights of sleep analyzed. Of 1409 separate time zone changes recorded, 720 were eastward travel, and 689 were westward travel. The mean results of each sleep quality and physiologic measurements captured by the WHOOP 3.0 device are outlined in Table 1. Frequency data of TZDs are represented in Figure 1, with a maximum of 11 but a skew towards ≤3 time zones traveled. The univariate analyses showed TZD and travel direction each had a significant influence on sleep duration. The resulting mixed linear regression model is depicted in Table 2 and Figure 2. Compared with average sleep of 436.8 ± 2.8 minutes without travel, on the first night after travel, for every hour of TZD, regardless of direction traveled, players would sleep 11.3 ± 1.0 minutes less (P < 0.05). Eastward travel further reduced sleep (−24.5 ± 4.2 min; P < 0.05), while westward travel increased sleep duration (+30 ± 4.2 min; P < 0.05). Multivariable analysis of physiologic parameters identified several statistically significant correlations in RR, RHR, and HRV but with small effect size, and no significant correlations were found for HRV-CV, skin temperature, and SpO₂. The statistically significant correlations are summarized in Table 4.

Figure 1.

Histogram displaying the frequency of travel events for each TZD in hours. TZD, time zone difference.

Table 2.

Mixed linear model prediction of athlete sleep duration after travel across x time zones on nights 1, 2, and 3, respectively ^a

	Night 1	Night 2	Night 3
Total sleep duration with no time zone change, minutes	436.8 ± 2.8	433.4 ± 2.8	433.7 ± 2.8
Change per time zone difference, minutes	−11. 3 ± 1.0	−2.1 ± 1.0	−2.6 ± 1.0
Eastward travel, minutes	−24.5 ± 4.1	+12.9 ± 4.3	+3.5 ± 4.4 b
Westward travel, minutes	+30.0 ± 4.2	+28.7 ± 4.5	+19.2 ± 4.5

Sleep duration reported as mean ± SE.

^aApplication of the model is detailed in Table 5.

^bUnivariate analysis found statistical significance (P < 0.05) in all variables below except eastward travel on night 3.

Figure 2.

Visualization of the relationship between TZD in hours and sleep duration in minutes on the first night after travel. Blue line indicates the influence of TZD regardless of travel direction. The red and purple lines indicate the influence of TZD with westward and eastward travel direction, respectively. The gray areas around each slope indicate 95% CI. For reference, the gray dashed line shows a 7-hour (420 minutes) sleep duration. TZD, time zone difference.

Table 4.

Statistically significant relationships between travel and physiologic parameters identified by multivariable analysis

Travel variable	Significant effect on physiologic parameter
TZD	• RR increases by 0.01 breaths per minute per TZD on night 1
Eastward travel	• RHR increases by 0.4 bpm on night 1 and 0.5 on night 2 • RR increases by 0.05 breaths per minute on nights 1, 2, and 3 • HRV increase by 2.2 ms on nights 2 and 3
Westward travel	• RHR increases by 0.5 bpm on night 1 • RHR increases by 0.8 bpm on night 2

bpm, beats per minute; HRV, heart rate variability; RHR, resting heart rate; RR, respiratory rate; TZD, time zone difference.

Influence of Sleep/Recovery on Match Performance

A total of 49 players and 1707 matches were included in the analysis of sleep, and recovery on match performance. Of these matches, 81.3% (1388 matches) analyzed were played ≥3 nights after the athletes’ arrival at the tournament location. In the results of the univariate analyses, several models revealed statistically significant relationships, although all with seemingly small effect sizes; these data are summarized in Table 3. For models that produced significant results, these sleep/recovery variables were analyzed together in a multivariable linear mixed model to determine the influence of multiple sleep/recovery parameters on a single performance metric, as noted in Table 4. This analysis did not result in any significant correlations. Subgroup analysis of (18.7%) matches played within 3 nights of arrival at the tournament location did not result in significant correlations.

Table 3.

Statistically significant relationships identified by univariate analyses of travel, sleep, and physiologic parameters, and match performance

Match performance statistics	Significant relationships
Total points	• Rank difference (B = 0.005) • Sleep consistency 2 nights before match (B = 0.032)
Unforced errors	• Sleep consistency 3 nights before match (B = 0.02)
Double faults	• Respiratory rate 2 nights before match (B = 0.183)
Number of aces	• HRV 2 nights before match (B = 0.006) • Single rank (B = 0.001)
Total winners	• HRV 2 nights before match (B = 0.011)

B, coefficient of change; HRV, Heart rate variability.

Discussion

In this study, we used the wearable biometric capture device WHOOP 3.0 to collect sleep physiologic data of elite professional women tennis players and analyzed these against travel across time zones and match performance. For our primary aim of evaluating the impact of circadian desynchronization on sleep parameters, we were able to identify a near-linear correlation between time zones traveled and duration of sleep on night 1 after travel (Table 2). However, when we analyzed time zone difference against physiologic measures, correlations are inconsistent, and in cases where a statistically significant correlation does exist, the effect size tends to be small and clinically irrelevant. For our secondary aim of correlating sleep and physiologic data with match performance, we were unable to identify statistically significant results that had clinically meaningful correlations.

The findings from our study highlight the impact of travel direction and time zone change on sleep duration, being most prominent on the first night after travel, particularly with eastward travel. This effect was observed to taper down by the second and third nights, consistent with previous research that demonstrated sleep disruption was most affected within the first 48 hours after travel.^36,39 Based on these findings, logistical guidance for players may be to arrive at the tournament destination ≥3 days before match play to acclimate to the new local time zone.

In addition, both easterly and westerly travel can cause sleep disruption in separate ways. Eastward travel is generally accepted as more disruptive to circadian rhythms as it advances the sleep schedule, which is more challenging for the body to initiate sleep, compared with delaying sleep with westward travel.^15,18 Our study’s linear mixed model illustrates that sleep duration is generally extended 30 minutes by westward travel. This is likely because lengthening of the day allows players to naturally extend their sleep period, fall asleep earlier in the new time zone, and have less trouble adjusting when traveling in this direction. However, westerly travel may be associated with earlier morning wake times. ²

Several studies have established an association between the number of time zones crossed and circadian desynchronization, negatively affecting physiological functions, sleep, and performance measures.^5,23,35 Our findings indicate the number of time zones crossed has a dose-response effect on sleep duration, with the linear models showing that a 2-hour TZD reduces sleep duration <7 hours (420 minutes) on the first night. At least 7 hours of sleep per night has been established by the American Academy of Sleep Medicine and Sleep Research Society as sufficient sleep duration for healthy adults. ⁴⁵ Previous studies in athletic populations, observed that when sleep is reduced to less <7 hours, cognitive performance (including alertness, reaction time, memory, and decision-making), as well as physical performance and injury risk, are negatively impacted.^5,23,24 The influence on subsequent days consistently decreased. This finding is helpful for understanding the threshold at which TZDs begin to impact sleep quality. A notable limitation is that we did not include nap duration in the calculation of total sleep time. While we recognize the role of strategic napping as a tool to alleviate physiological stress caused by travel and jet lag, our analysis was specifically designed to assess the impact of jet lag on nightly sleep quality parameters.

Using our model, Table 5 presents 2 travel scenarios to illustrate the impact of TZD and direction on predicted total sleep duration. According to our data, TZD alone was expected to reduce total sleep duration by 11 minutes of sleep per hour on the first night, so a 2-hour TZD is −22 minutes of sleep. Typical nontravel sleep is 437 minutes; thus, the player could expect 415 minutes of sleep (<420 minutes [7 hours]) on the first night of arrival. This sleep deficit is exacerbated if the travel was eastward (−24 minutes). As westward travel increased sleep by +30 minutes, it would take up to 4 TZD to result in sleep duration <7 hours, whereas traveling eastward (−24 minutes) is more vulnerable after just 1 TZD.

Table 5.

Calculation of sleep duration on first night of arrival with different travel scenarios

	No travel	Eastward travel a	Westward travel b
Average sleep duration, minutes	437	437	437
TZD c	0	−22	−22
Directional influence, minutes	0	−24.5	+30
Net change, minutes	0	−47.5	+8
Sleep duration after TZD and direction, minutes	437	390.5 d	445

TZD, time zone difference.

^aTwo hours TZD in the eastward direction.

^bTwo hours TZD in the westward direction.

^cEleven minutes per hour TZD × 2 hours.

^dIndicates a <7-hour threshold.

Regarding physiologic parameters, our multivariable analyses found a few statistically significant correlations although with small effect sizes (Table 4). Increases in RHR and RR are both consistent with a stress response, and the pattern that stress effect may peak on nights 2 or 3 rather than on the first night after travel is consistent with previous literature findings,^3,11,41 perhaps highlighting the role of accumulated fatigue due to jet lag. ⁴⁴ On the other hand, we observed a small positive correlation in HRV on nights 2 and 3 after travel, which contradicts our hypothesis that HRV would decrease as a stress response to travel and jet lag. These marginal magnitudes of the physiological change may be explained by the fact that the majority of trips analyzed involved crossing <3 time zones - the minimum generally required to induce clinical jet lag. ¹⁸ In addition, our study does not account for confounding environmental factors such as temperature, humidity, and altitude at the tournament destination; or athlete chosen mitigating factors such as naps; which may also have direct effects on physiologic parameters.

For 81.3% of the matches analyzed, the player had ≥3 nights at the destination before the match. As reported, the impact of travel and direction on sleep duration is most pronounced on night 1 and lessens the subsequent 2 nights. Therefore, it can be inferred that the longer an athlete is in their destination time zone before match play, the less impact the previous travel would have on their sleep duration. If perhaps more athletes had matches played within the first 48 hours after arrival, we would have more data to analyze the impact of travel, sleep quality, and match performance during a heightened period of risk for circadian desynchronization. While the exact reasons may vary for each athlete arriving ≥3 nights before match play, such as physiotherapy treatment, acclimatization, practice or flight schedule, etc, it is likely that early arrival also benefits them to correct circadian desynchrony.

Previous studies that have investigated the effects of sleep disturbance after travel on athletic performance have found decreased performance, such as decreased observed grip strength in British rugby league players. ³² However, to our knowledge, no studies have explored the association between disrupted sleep parameters after travel and specific tennis performance. In our study, while there was a statistically significant relationship between some performance metrics, such as total points won and unforced errors, with specific sleep parameters like sleep consistency, the small effect size suggested minimal practical relevance in real-life scenarios (Table 5). Moreover, there was no pattern established between variables and likely indicate the confluence of multiple confounding factors of competition, environment, and simple chance. Although our results did not definitively indicate that sleep duration directly impacts match performance, it is still advisable for athletes to arrive ≥3 days before match play to correct circadian desynchrony for overall health and wellbeing.

Subsequent research should seek to combine data from wearable biometric capture devices along with other contextual elements (environmental and individual) to deliver a more comprehensive understanding of how travel and sleep interacts with the overall sporting environment including practice and training sessions.

Players on the WTA Tour provide a unique study population as the WTA Tour offers players over 50 tournaments in 30 countries and 6 continents across the world, in addition to 4 Grand Slam events, in which players can choose to participate. To aid in travel logistics and court-surface consistency for the health of the players, the tournament calendar is arranged by region or “swings” including the Australia/New Zealand swing (hard court), the European swing (clay and grass courts), US swing, and Asian swing (hard court). With the implementation of the AER/PDP in 1995, the WTA Tour provides players guidance for selecting their personal tournament schedules, especially for newer players on tour, but, ultimately, the player and her team determine her travel and tournament schedule. This is quite a distinctive situation compared with many team-based sports who travel together during a limited season in a smaller region or country. However, a limitation of our study is that >50% of matches analyzed occurred after only a 1-hour TZD. This admittedly limits the applicability of our results to longer travel durations. Due to the purposeful regional schedule of WTA tournament “swings” and the selected schedule of a player, most matches analyzed involved players crossing only 1 to 2 time zones, which may not capture the significant disruptions of transcontinental or intercontinental travel. To fully understand the impact of extensive travel on sleep and performance, future studies should include a broader range of time zone changes.

Another limitation of our study affecting analysis of both physiologic parameters and performance measures is that we did not account for strategies that athletes already use to mitigate jet lag, including the potential use of permitted pharmacologic treatments, such as melatonin, or other nonpharmacological sleep aids that help realign the circadian rhythm to a new time zone and are common remedies for jet lag. ⁴³ This is an uncontrolled confounding factor that may have impacted our results. A systematic review of interventions to manage jet lag in athletes found low quality evidence for specific recommendations, but did note that resynchronization of circadian rhythm may be expected to take 1 full day (1 hour per day) for eastward travel and 1 half day (2 hours per day) for westward per TZD, which is supported by our data as well. ¹⁸ We were also not able account for the effects of social jet lag,^4,28 where the circadian rhythm is impacted by sleep patterns for work (training/competition) days versus rest days - a factor that may continue to influence athletes well into the tournament, beyond the initial impact of travel-related jet lag.

Timing of matches, in particular late-night matches, are likely to impact sleep consistency due to the marked adjustment of bedtime. For certain players, there may be postmatch obligations such as media, antidoping procedures, and player recovery modalities. Subsequently, the following match start time may not take into consideration the end time of the previous match, leading to less opportunity for rest and recovery. In recognition of potential negative outcomes from late night matches, in 2024, the WTA issued a new “late night match policy.” This policy states the latest a match can be held on court is 11:00 pm, with a change of court being made no later than 10:30 pm. In addition, for protection of younger players under the age of 16 years, and appreciation of longer sleep requirements for adolescents, the policy states the tournament is to make best efforts not to start their match after 9:00 pm. Further studies may consider evaluating the impact of match times on sleep and performance.

Several pragmatic guides exist that discuss management strategies, specifically for athletes, to help mitigate the negative effects of circadian dysregulation associated with travel.^2,18 The first step to improved sleep is education for athletes and their support teams about the value of sleep for health and performance. Practical tips are broken down to pre-, during, and post-travel. Pre-travel suggestions include protected sleep, banking sleep, and avoidance of sleep debt. During-travel recommendations focus on hydration, sleep while traveling, and illness protection. Post-travel guidance highlights the importance of resynchronization with timing of exercise/training with natural light exposure, timing and composition of meals, use of melatonin as needed, sleep hygiene interventions, and potential use of caffeine to decrease daytime fatigue.^2,8,19,20,34 Proper education and guidance on sleep hygiene has been shown to increase sleep duration during tennis tournaments. ²⁵ Increasing sleep duration, either with 20- to 90-minute naps or nighttime sleep, has been shown to improve mental and physical performance. ⁹ Our study adds to this guidance by suggesting a model for predicting sleep duration changes based on direction of travel and TZD. It should be noted, however, that our sleep analysis was limited to a maximum of 3 nights after travel, whereas it may require >3 days to achieve full entrainment after distant travels. Findings from our study can be used in a practical manner when planning travel for these tennis players or extrapolated for other sports teams/athletes that may travel internationally as well. Screening for sleep problems should become standard for individual and team sports so that intervention can begin. ² The most frequently used screening questionnaire is the Athlete Sleep Screening Questionnaire (ASSQ), which has been validated and offers recommendations based on level of sleep difficulty. ³³

To summarize, limitations of our study include the inability to account for environmental factors such as temperature, weather, and altitude, all of which may influence physiological parameters. In addition, we were unable to account for the individual mental preparation strategies athletes employ to manage the stress resulting from travel fatigue and jet lag; such as strategic napping, mindfulness, and arrival time at tournament. As the game of tennis has a large psychological component, the importance of mental fortitude and its impact on match play, resiliency, and travel fatigue/jet lag recovery cannot be understated.

Conclusion

In this unique population of female professional tennis players on the global WTA Tour, we demonstrate the impact of travel induced circadian desynchronization on markers of sleep and recovery as recorded by WHOOP 3.0. Notably, sleep duration reduction was most prominent on the first night after travel, particularly with eastward travel, but improved on subsequent nights. Our findings indicate the number of time zones crossed and direction may predictably influence sleep duration. This valuable information may be translated into practical guidance for athletes and their teams while planning travel; to allow appropriate adjustment to the new time zone with the goal of optimizing sleep, recovery, and, ultimately, performance.

Clinical Recommendations

• Sleep is imperative for overall physical and mental health, injury risk, recovery, and athletic performance (SORT Level A).

• Circadian desynchronization may occur with travel across time zones, particularly with eastward travel (SORT Level A).

• Sleep duration may be predicted based on time zones crossed and direction of travel (SORT Level B).

• Sleep disruption may impact athletic performance. (SORT Level C).

• Athletic travel across time zones should be planned with destination arrival several days in advance of competition to account for circadian resynchronization and recovery (SORT Level C).